Additional material 1A: Historical and archaeological information
Las Isletas
Directly to the west of Las Isletas peninsula lies Granada (population 100,000) founded in 1534 by the Spanish. There is no historic record of this collapse event in the city’s colonial reports, indicating it occurred prior to 1534. Neither the amphitheatre nor the deposits have been covered by later magmatic activity, thus it seems the event took place after Mombacho’s latest resurfacing episode.
Extraordinarily, we found human bones and ceramics in the southern extremity of Las Isletas peninsula at the base of a hummock formed by a mix of soil and blocks (Fig. a1 and Fig. 3 in main text for location). Traditionally in pre-Columbian populations (before the Spanish conquest), people were buried and later dug up (a few months to a few years later in order to allow decomposition). Afterwards the bones were enclosed in an orderly fashion into an egg-shaped urn, arms and legs crossed (Fig. a1b).
Numerous elements of these egg-like urn fragments have been found dispersed in this particular deposit mixed with bones, pottery and silica tools. In one outcrop, half a burial urn was found cracked, surrounded by fragments of skull, tibias, femurs, jaw, arms, all in an extremely well preserved position considering the type of transport they probably suffered (Fig. a1c, d, e, f). Similarly to the avalanche material, these archaeological fragments have jigsaw-like cracks. The nature and position of these bones, associated with these burial urns suggests that an ancient cemetery was remobilized by the avalanche. Although this deposit encloses large blocks from the avalanche deposit, the matrix is almost entirely made of well developed soil. In most regions of Las Isletas DAD, soil development reaches 20-40 cm in depth. Thus this soil and cemetery was probably formed before the northern collapse and then entrained by the avalanche. Accordingly, dating these bones will not give an age representing an avalanche victim’s death but rather a native’s “natural death” and burial. Nevertheless carbon-dating on these bones will be done to constrain the earliest possible date for collapse.
INSERT FIGURE a1 HERE
Fig. a1: (a) Bone and pottery locations in the Las Isletas peninsula on orthophotographic map (INETER). Note how they outcrop on both sides of this particular hummock. (b) Photograph of an intact pre-Columbian burial urn (above) and sketch of how bones were generally positioned (below). Urn ≈ 60-70 cm in length. (c) Outcrop of corresponding bone location with bone type, position and surrounding ceramic fragments. (d) Enlargement of skull area, note the well preserved original cranial shape. (e) Void left by half of a burial urn after removal of its fractured pieces. Again, original egg shape is preserved. (f) Arm bones (the lower one has roots growing inside), jaw, teeth and tibia.
Additional material 1B: Historical and archaeological information
El Crater
Fortunately, reliable historical information from the Spanish colonies of the 16th century exists. The city mentioned in the archives is always Granada, as it was the first and only colony in this region at that time (see translations in additional material 1C).
The three archives lead us to the following conclusions:
- Las Isletas and El Crater avalanches were not synchronous; the latest was probably El Crater as the Las Isletas event has not been historically documented.
- Archives describe the collapse of the “summit” of Mombacho, which supports the hypothesis that not only materials filling today’s amphitheatre collapsed but also an additional uppermost fraction.
- The first archive mentions a strong “wind and water storm” prior to the collapse which can relate to the typical tropical heavy rainfalls or to the recurrent hurricane storms which still occur today.
- There was strong and frequent seismic activity in the days preceding the collapse, and probably earthquake activity that started the night before.
- There is no description of any magmatic activity associated with the collapse
- An Indian village was destroyed, together with 400 inhabitants.
- The avalanche seems to have generated some type of “water and rock storm” that could be attributed to the emptying of water reservoirs on the avalanche path. The avalanche travelled over a low plain that probably contained swamps and lakes, as today. Another possibility is that a crater lake was present at the summit and was emptied by the collapse.
- The nearby Masaya volcano was under more intense surface magmatic activity than it is today with possible related volcano tectonic deformation (Girard and van Wyk de Vries, 2005).
Unfortunately we failed to find archaeological traces of the buried village mentioned in these archives.
It is clear that the current vegetation is less dense over the southern DAD than it is over the northern DAD, and that the tree forest is younger. This supports the relatively young age of El Crater compared to Las Isletas.
Additional material 1C: El Crater historical accounts
Description of the province and government of Nicaragua, by Joan Lopez de Velasco.
[…] Some volcanoes are located close to the city, in particular the Masaya, which is notable for its size and depth: We can climb up to its summit on horse as it isn’t very high; Its mouth has a circumference greater than a half league [note: the league is equivalent to 4828 m], and shows a depth of more than two hundred brazas [note: the braza is an ancient Spanish measurement and is equivalent to 20.112 m]. […] The fire that can be seen inside moves with great noises and produces “fire waves” comparable to seawater, it seems like it could be some kind of liquor or metal. Occasionally it throws some bubbles of the same material, which stay in the small plain where the first and deepest mouth is found. Some have attempted to fill vases and cauldrons with this boiling liquor, but the fire consumes and dissolves everything.[…]
Four leagues away from the city there was an Indian village named Mombacho, close to a smaller volcano, which one night collapsed during a big water and wind storm in the year 70 [note: 1570], and a whole side fell onto this village. None escaped but one neighbour from Granada named Caravallo, and some old Indian women, a total of six or seven Spanish were buried with them. Furthermore, such a great storm of water and rocks was produced, that it damaged greatly the cacao plantations and farming animals throughout a distance of six leagues.
Visit of Fray Alonso Ponce to the province and district of Nicaragua, in 1586, by Antonio de Ciudad Real.
[…] A league from Granada, at its southern coast, stands the Mombacho volcano, which, in the past years, collapsed in the region of the southern sea, and expelled so many rock mounds that a village of four-hundred Indian neighbours was devastated, with only one person escaping. This person, having seen the big preceding earthquakes, and not knowing what they meant, went to inform the Spanish in Granada, and meanwhile the collapse occurred. If this was to occur in the region of the lake situated at volcano’s northern coast, the city of Granada would be devastated and destroyed. Old Spanish people from the city told the regional delegation that few days before the volcano collapsed, the earth was shaking frequently and that the night prior to the collapse the neighbouring plains and prairies were trembling and moving, just like the lake’s waters move during the storms. They also said that in Granada, almost all houses lost their roof tiles, and that many walls and houses fell. At the moment the earth doesn’t shake anymore. The chief delegate being in Granada currently, he will be witnessing any further event.
Note n°50 of the Geórgicas written by Juan de Guzmán, 1586
[…] The other volcano I mentioned, the Mombacho, is one day away from here, near the city of Granada, in which there was an old woman, who came from an Indian village, near the shores of the nearby lake forming a fresh water gulf. Near this Indian village there is a high and powerful mountain, named the Mombacho after a village which was located in its surroundings. One night, noise came out of the mountain, which frightened the Indians greatly, who started leaving the village to other places. The old woman, seeing what was going on, left to advise her daughters and their babies, who lived in the village at that moment. One morning, two hours before sunrise, the summit of the mount was torn and fell onto the village, and consequently destroyed and covered it with so much soil and rocks, that it is today invisible. In this way they all were buried, until judgement day when they resuscitate, without one escaping, there is nothing else to understand but the village’s sudden death. If the same fraction of the mount was to fall from the other side, surely the city of Granada would be buried in the same way. [...] A great quantity of water also accompanied the collapse, which gave a powerful hit towards the other side of the coast, close to the road taken by the Spanish when they go from there to Leon, and carried the strong palm trees as though they were straws. In the summit of the mount was left an opened mouth, from which smoke comes out every morning.
Additional material 2A: Hummock Analysis at Las Isletas
Distinctions can be made when considering hummock morphology. When composed of block-rich units, hummocks display high slope angles, sharp summits, and have forest cover. Conversely, when composed of block/matrix units, they show gentler slopes, rounder summits and are covered by grassy vegetation. In the most easterly zones of the Isletas peninsula, hummocks are occasionally formed by basal lapilli-rich deposits, and have similar gentle slopes. Occasionally, mounds enclose loose scoria, and show even smoother topography. Finally, areas dominated by basal pumice-rich units never show hummocky surfaces; instead they show depressions where water often accumulates. A statistical analysis of the hummocks is given herein.
Generally, hummocks vary in length, width, shape, height, and in their elongation direction. A statistical examination of Las Isletas was carried out on 830 hummocks manually contoured using orthorectified aerial photos. In this way precise length, width, and area data were obtained, which were further grouped by dominant lithology A4 (matrix and blocks) or A5 (mostly blocks). Orientations were computed by a simple least-square ellipse fitting method applied on the perimeter of each hummock. Because simple 2D mapping does not allow characterization of height, a 1/10000 scale topographic map of the Las Isletas peninsula area was used to obtain 480 height measurements. As we will illustrate, hummocks formed by matrix-rich units (A4) are generally smaller and show less topography than hummocks formed by block-rich units (A5). Because the available topographic map only shows the higher hummocks, height measurements were not grouped into different units, as this left too few A4-formed units for robust statistical analysis. Results obtained from orthophotos are shown on Fig. 8a, b, c, d in the main text and Fig. a2a, b, c, d and height measurements are shown on Fig. a2e. The simple histograms are paired with cumulative frequencies to allow the evaluation of individual contributions. The different plots indicate that:
hummocks do not show a dominant orientation (Fig. 8a, b in the main text) and are unevenly distributed around the avalanche flow direction. This is confirmed on the cumulative frequencies plot (Fig. 8b) where the corresponding A5 distribution curve shows an equal participation of all lengths (almost a straight line). A4 does not show any significantly stronger participation either but suggests a minor participation of the 30-110° range.
lengths both for A4 and A5 show a non-normal distribution of the data with positive skewness (longer tail to higher values). Both the histogram and the cumulative plot (Fig. 8c, d in the main text) show a stronger component of longer hummocks in A5 than in A4. This is well seen on the cumulative plot where the curve representing A4 is shifted to the left of A5 (greater participation of shorter hummocks). The average value for A4 hummocks is 86 m and 100 m for A5 hummocks, whereas their maximum occurrences are located around 60m and 80m respectively.
widths (Fig. a2a, b) also follow a similarly skewed distribution with again a slightly higher participation of narrower hummocks in A4 (upper cumulative curve for A4 shifted to the left of A5). Average widths are 54 and 60 m for A4 and A5 hummocks respectively, and highest participations are around 60 m for both.
areas covered by individual hummocks (Fig. a2c, d) shows positive skewness as well, and although the relation between A4 and A5 is not clearly visible on the histogram, the cumulative curves show the same configuration as before. Thus as expected, if hummock lengths and widths are smaller in A4 than in A5, area follows the same trend. Average hummock area is 5630 m2 for A5 and 4210 m2 for A4. Maximum area occurrence is approximately 3000 m2 for A4, here significantly larger than A5 (2000 m2).
width vs length plots for A5 and A4 hummocks (Fig. 8 e, f in the main text) show a positive correlation and illustrate how data density is higher for shorter and narrower hummocks. They also show a tendency to concentrate more along the ‘length=width’ line (circular base hummocks) which confirms a significant proportion of non-elliptical hummocks.
similarly to the previous histograms, heights follow a positively skewed non-normal distribution, with a 9.25 m average and 9 m maximum occurrence value (Fig. a2e). The length vs height plot (Fig. a2f) shows hummock height is grossly positively correlated to length.
Additional material 2B: Hummock Analysis at El Crater
Similarly to Las Isletas, a statistical analysis was made on hummocks covering El Crater. Using orthophotos, our mapping allowed more than 2900 hummocks to be measured in length, width, area and orientation. Unfortunately no available detailed topographic map covers the southern avalanche, thus hummock height could not be measured.
Results are plotted on Fig. 8g, h, i, j, k, l in the main text and Fig. a2g, h, i, j, along with those from Las Isletas to allow clear comparison. Histograms and plots show that:
the data defines a dominant orientation of approximately 140-150° (Fig. 8g, h in the main text). This orientation does not completely parallel avalanche flow direction which is generally oriented N-S (~170°). Both hummocks formed by B2 and B3 have this direction, which confirms it is not an artefact generated by one of the datasets. The cumulative curves for both types of hummocks show a minor participation of 30-100° orientations.
hummocks formed by B2 are significantly shorter than those formed by B3 (Fig. 8i, j in the main text); however, both show the same typical skewed distribution previously described for Las Isletas. The distribution curve for B2 is significantly shifted towards the lower values and shows strongest participation of 40-80 m long hummocks. Average length is 77 m for B3 and 56 m for B2 with maximum frequency around 60 m and 75 m respectively.
widths follow the same pattern as length, with an even higher shift of B3 hummocks toward higher values (Fig. a2g, h). Average values are 47 m for B3 and 36 m for B2 with maximum occurrences at 40 m and 60 m respectively.
areas covered by individual units are better distributed than Las Isletas, with a clear difference between B2 and B3 hummocks, the latter covering more ground (Fig. a2i, j). Average area is 3200 m2 for B3 and 1835 m2 for B2 whereas 2000-2400 m2 and 1200-1600 m2 are the most frequent values respectively.
length vs width plots (Fig. 8k, l in the main text) are indistinguishable for both units and similarly to Las Isletas, a positive correlation is observed with an increasing scatter toward higher values. Data density is also higher towards the ‘length=width’ line suggesting significant participation of circular hummocks in the data.
INSERT FIGURE a2 HERE.....
Fig. a2: Las Isletas (a to f) and El Crater (g to j) hummock statistical analysis. The rest of the plots are shown in Fig. 8 in the main text. Histograms and corresponding cumulative frequency curves of A4/A5 and B2/B3 hummocks are shown for: width (a, b, g, h) and area (c, d, ,i, j). Global hummock height distribution at Las Isletas (e), and height vs length (f) plots.
Additonal material 3: Description of La Danta (or Santa Julia) DAD
A third DAD is observed to the southeast of Mombacho, hummocky topographies allowing it to be easily recognized on satellite images. The collapse volume cannot be determined since the corresponding amphitheatre is now absent, most likely filled by later lava units and tephra. A small east-west orientated scarp could be the only remnant of the generated scar (Fig. 2 in main text). Hummock dimensions appear similar to those of El Crater avalanche, smaller than those observed at Las Isletas. Its runout can not be evaluated with acceptable precision as the distal end of the deposit may lie in Lake Nicaragua. Width measurements are also compromised by El Crater DAD and other deposits lying over its western limits. Chronologically, La Danta avalanche clearly occurred before El Crater avalanche as the latter lies on top of the former. The surface of La Danta DAD is also much more weathered and there is a greater soil development than at the other DADs. As Las Isletas avalanche and corresponding amphitheatre were not significantly covered by later magmatic products, we infer La Danta avalanche is older and occurred before the last major edifice building eruptions that have produced the fresh lava flow topography on the volcano.